The present invention is directed to a method for recovering hydrocarbons (bitumen) from tar sand deposits. According to the process, hydrogen sulfide is generated by upgrading bitumen produced from minable tar sands and hydrocarbons are recovered from an unminable subterranean tar sand formation in an area proximate to the area of the hydrogen sulfide recovery by utilizing an improved in-situ combustion operation wherein the hydrogen sulfide is injected into the subterranean formation prior to commencement of combustion.
Increasing worldwide demand for petroleum products, combined with continuously increasing prices for petroleum and products recovered therefrom, has prompted a renewed interest in the sources of hydrocarbons which are less accessible than crude oil of the Middle East and other countries. One of the largest deposits of such sources of hydrocarbons comprises tar sands deposits found in Northern Alberta, Canada, and in the Midwest States of the United States. While the estimated deposits of hydrocarbons contained in tar sands are enormous (e.g., the estimated total of the deposits in Alberta, Canada is 250 billion barrels of synthetic crude equivalent), only a small proportion of such deposits can be recovered by currently available mining technologies (e.g., by strip mining). For example, in 1974 it was estimated that not more than about 10 percent of the then estimated 250 billion barrels of synthetic crude equivalent of deposits in Alberta, Canada was recoverable by the then available mining technologies. (See SYNTHETIC FUELS, March 1974, Pages 3-1 through 3-14). The remaining about 90 percent of the deposits must be recovered by various in-situ techniques such as electrical resistance heating, steam injection and in-situ forward and reverse combustion. In addition to tar sands, heavy, viscous crudes and crudes from partially depleted reservoirs are also recoverable by in-situ production techniques.
While details of operating of all of such in-situ techniques vary, a common objective thereof is to lower the viscosity of the hydrocarbon deposits to the point where they can be pumped to the surface of the formation with equipment normally available at the formation site.
Of the aforementioned in-situ recovery methods, in-situ combustion (both forward and reverse) appears to be the most promising method of economically recovering large amounts of hydrocarbon deposits with currently available technology. The attractiveness of the in-situ combustion methods arises primarily from the fact that it requires relatively little energy necessary for sustaining combustion of the hydrocarbon deposits. In contradistinction, other in-situ techniques, such as electrical resistance heating and steam injection require considerable amounts of energy, e.g., to heat the steam at the surface before it is injected into the petroliferous formation.
Conventional in-situ combustion involves drilling of at least two substantially vertical wells into the formation, the wells being separated by a horizontal distance within the formation. One of the wells is designated an injection well, and the other a production well. The recovery of hydrocarbons is accomplished by raising the temperature around a bore hole to the combustion temperature of the petroliferous deposit with some type of a conventional down hole heater/burner apparatus, and then supporting the combustion by injecting an oxidizing gas, e.g., oxygen or air into the formation. There are two basic processes of in-situ combustion, viz., forward and reverse combustion. Forward combustion is initiated at the oxidant injection well and the combustion front propagates toward the production well. Reverse combustion is initiated at the production well and the combustion front propagates toward the oxidant injection well. Hydrocarbon vapors produced during the combustion process are recovered at the surface of the formation and stored in appropriate containers. The combustion is conducted at a temperature not to exceed 1500.degree. F. for about 12 months until the viscosity of oil deposits is reduced to 700-800 cp, generally considered necessary for pumping the oil to the surface of the formation. Further details of forward and reverse in-situ combustion techniques are set forth in SYNTHETIC FUELS, March 1974, pages 3-4 through 3-14, and in THE TAR SANDS OF CANADA by F. W. Camp, pages 27-34, Cameron Engineers, Inc., Denver, Colo., 2nd Edition (1974), the entire contents of which are incorporated herein by reference. Modified in-situ combustion techniques using a combination of oxygen and other chemical substances are also known in the art. For example, Heilman et al., U.S. Pat. No. 2,718,263 uses a mixture of oxygen-containing gas and fuel to generate heat in the formation, and Elzinga, U.S. Pat. No. 3,087,541, injects fuel into the formation only after the combustion has started. Both of these modified in-situ prior art combustion processes uses fuels injected externally into the formation either simultaneously with oxygen or after the injection of oxygen to control the direction of speed of propagation of the combustion front.
However, heretofore practiced in-situ combustion techniques have resulted in a relatively low rate of recovery of available hydrocarbons from subterranean petroliferous formations. For example, the rates of recovery have been reported to be less than about 50 percent of the total deposits of tar sands, e.g., SYNTHETIC FUELS, March 1974, pages 3-4 through 3-14.
Tar sand formations with an overburden greater than 500 feet are not amenable to recovery by surface mining. Such formations present a problem as to ways and means of economic recovery. The techniques available for such recovery are limited and are restricted to in situ techniques. Many in situ techniques have been considered such as electrical resistance heating, steam injection and fire flooding. Inherent problems exist in all in situ methods, for example, poor volumetric sweep, fluid displacement profiles in steam injection and difficulty in starting combustion and maintaining design temperatures in fire flooding. Other tar sand formations that do not have such a thick overburden are amenable to surface mining techniques. Usually the overburden is removed and the underlying desirable tar sand is scooped up and processed for hydrocarbon recovery. Many procedures have been described for this separation of which the Clark Hot Water process is typical.
As recovered the bitumen is not usuable and is usually upgraded by delayed or fluid coking. This coking step produces a liquid product which is usually fractionated into three products. Each of these products is further upgraded by hydrogen treatment. As a result of this upgrading step, the sulfur usually contained in the products is converted to hydrogen sulfide.
Hydrogen sulfide has very limited or no use as a product and usually is converted to elemental sulfur by way of the Claus process. In certain distant locations it may not be possible to sell the sulfur and a large inventory of unsold sulfur could result. It is thus desirable to minimize the amount of sulfur generated on such sites.
Thus, the location of a tar sand recovery plant and its upgrading facilities could be used to advantage so as to minimize the production of sulfur. By selecting a tar sand deposit amenable to surface recovery adjacent to a deposit which is not and which requires the use of in situ production techniques, this desirable minimization of the production of sulfur could be achieved.
In my copending application, Ser. No. 260,521, filed May 4, 1981, there is disclosed an improved in-situ combustion process for the recovery of viscous oil from tar sand formations wherein a combustible gas such as a relatively light hydrocarbon gas having a condensation point of -173.degree. C. to -43.degree. C. or hydrogen sulfide is injected into the formation prior to the initiation of combustion.
The present invention is an improvement of the process disclosed in my copending application described above wherein the hydrogen sulfide injected into the formation prior to initiation of in-situ combustion is produced on site as a by-product in the upgrading of minable tar sands. Therefore, the present invention allows recovery of difficult to produce tar sand formations and at the same time uses available hydrogen sulfide thus minimizing the amount of sulfur produced as a by-product of hydrogen upgrading of products produced from minable tar sands.